This invention relates generally to color television picture tubes and particularly to a tensioned shadow mask/frame assembly for such tubes.
FIG. 1 shows a cross section of a color picture tube (kinescope) in which the invention can be utilized. The tube 10 includes a funnel portion 11 the wide end of which is enclosed by a faceplate panel 12 and the narrow opening of which is enclosed by a neck portion 13. A phosphor screen 14 is arranged on the inside surface of the faceplate panel 12. The screen 14 is composed of triads of phosphor materials which individually emit the three primary colors of light when impacted by electrons. An electron gun 16 is mounted in the neck 13 and provides three electron beams which are scanned across the screen 14 to energize the phosphors and produce the desired display. Each of the three beams impacts a different color emitting phosphor so that the full range of colors is produced on the screen 14. A shadow mask/frame assembly 17 is supported by studs 18 a preselected distance from the phosphor screen 14. A shadow mask 19 is part of the mask/frame assembly 17 and includes a large number of apertures through which the electron beams pass to direct the various beams to phosphors of the proper light emitting colors.
One type of shadow mask is a tensioned shadow mask. A tensioned shadow mask is permanently attached to the upper and lower horizontal members of a support frame. The horizontal members run parallel to the horizontal (X) axis of the kinescope. The horizontal frame members are held together by side members which extend parallel to the vertical (Y) axis of the kinescope. The mask contains a large number of narrow slits which extend parallel to the vertical axis. While the shadow mask 19 is being permanently attached (for example, by welding) to the horizontal members of the support frame 20, the horizontal members are pressed inwardly toward one another to deflect them. Accordingly, the shadow mask is under tension as the horizontal members attempt to return to their undeflected positions when the pressure is relieved upon the completion of the attachment. The tensioning of the shadow mask is necessary to insure that the spacing between the screen 14 and the shadow mask 19 remains constant along the longitudinal (Z) axis of the tube 10. Despite the pretensioning of the shadow mask during its attachment to the support frame assembly, problems frequently arise with tensioned shadow masks. For example, when the faceplate 12 is frit sealed to the funnel 11 along the frit seal line 15, the tube is subjected to high temperatures, such as 600.degree. C. The shadow mask/frame assembly therefore expands and because high temperature and high tension (e.g. 20,000 psi) cause the shadow mask material to creep (become permanently longer), when the frame assembly returns to normal temperature the mask material is looser than it originally was. This results in a permanent change in the tension of the shadow mask and thus has an adverse impact on the quality of the display produced on the screen 14. Another difficulty frequently arises each time the tube is turned on. When the tube 10 is turned on, the frame and shadow mask are cold. The frame gradually heats up during the normal tube operation. However, the shadow mask has lower mass than the frame and therefore heats up and expands faster than the frame, this causes the tension in the shadow mask to temporarily decrease. In many instances the picture quality therefore is not preferable during the warmup period. For these reasons there is a need for a tensioned shadow mask/frame assembly which continuously applies tension to the shadow mask during all manufacturing and operating stages. The present invention fulfills this long felt need.